Kevin Wendell Crumb is the main antagonist of Split. He is a mysterious man who suffers from an extreme case of Dissociative Identity Disorder, and has 23 separate personalities residing in his mind. While Kevin himself is a good man, if even a little twisted, some of his other personalities, such as Patricia, Dennis, Hedwing and specially "The Beast", are malevolent individuals that take control of "The Light" (control over Kevin's body) most of the time.
He was portrayed by James McAvoy.
When Kevin was a child, his father left on a train and never came back. There is a fan theory that Kevin's father may have died in the same train crash (orchestrated by the villain Elijah Price, known by his unforgettable moniker "Mr. Glass") that left David Dunn, the protagonist of Unbreakable, as the sole surviving passenger. This left three-year-old Kevin at the mercy of his abusive mother, who would severely punish him if he did not keep everything clean and in order. To avoid his mother's abuse, Kevin began to develop multiple personalities as a defense mechanism. Dennis, the protector alter with obsessive-compulsive cleaning habits, was a direct result of his trauma.
Kevin ultimately developed 23 individual personalities, each of which are able to take control. If the fan theory involving Mr. Glass is true, then Glass not only created a superhero, David Dunn, but also inadvertently made Kevin a supervillain as well.
Kevin, presumably Dennis, worked at the Philadelphia Zoo when something jarring happened: two girls approached him and played a cruel, childish prank on him. Without warning, both girls took his hands and forced him to touch their breasts, before they ran away laughing. This left Kevin disoriented and in shambles, and possibly contributed to Dennis's peculiar sexual appetite.
Dennis And Patricia
As of September 18, 2014, the personalities seemed to have buried Kevin away deep in his mind. At the beginning of the film, Dennis (a stern man who suffers from OCD) kidnaps Casey, Claire and Marcia, knocking out Claire's father in order to abduct the three girls. He keeps them in a room in his home where the three girls are later introduced to Patricia (a matronly Englishwoman), another one of Kevin's alters. This bewilders and invokes fear within the girls. When Hedwig (a 9-year-old boy with a lisp) "takes the light" (control of Kevin's body), Casey tries to befriend him in hopes of escaping. Throughout the film, mainly through the discussions between Kevin's alters and his therapist, Dr. Fletcher, it is established that the presumed dominant personality is Barry (an effeminate male fashion designer) who tries to subjugate the others and keep them from staying in the light. However, two of the other personalities (Patricia, a religious nut, and Dennis, who also has ephebophilic fantasies) have suppressed Barry with the help of Hedwig, who is able to take the light at his will.
When Claire tries to escape the room through the drywall ceiling, Dennis finds her and then locks her in a separate room on her own. He tells her it isn't nice to trick children because it shows who someone really is. Soon after, when Patricia prepares food for Casey and Marcia, she forbids them from having Claire join them. When Marcia attacks Patricia and tries to make a run for it, Patricia finds her and threatens her with a knife. Like Claire, Marcia is put into a separate dingy room next to Claire's room.
Casey befriends Hedwig, and tries to influence him. Hedwig excitedly warns the girls about 'the Beast', a malevolent 24th personality with incredible abilities. He also reveals to Casey that there is a window in his room, which she perceives it as a potential escape route. However, when he shows her the 'window', it is shown to be a hand-drawn picture of a window.
She grabs a functional walkie-talkie nearby and radios for help, but the person on the other end thinks it's a prank and dismisses her; Patricia then subdues Casey. Barry sends several email messages for help to Dr. Fletcher, revealed to be Barry feeling threatened by the bid for control by Dennis and Patricia. When Dr. Fletcher approaches and tries to reason with Kevin, she realises that he has kidnapped the girls and tries to convince him that what he is doing is wrong. Dennis sprays her with chloroform and carries her into the living room. Enacting a sort of ritual, Dennis then buys flowers and Patricia visits a train car (presumably the same train Kevin's father left him at.) Dennis then becomes 'the Beast', exhibiting physical changes such as increased muscle mass and several inches added to his already tall height. The Beast snarls, his persona dissolving into an animalistic mentality before launching himself on a rampage. When she recovers, Dr. Fletcher writes down Kevin's full name, knowing that the resulting awakening of his original personality will be enough to subdue him. The girls, in the meantime, try to escape from their confinements.
"The Beast" Attacks
Returning to his hideout, “The Beast” confronts Dr. Fletcher. Before she is able to speak Kevin's full name, a tactic which would call Kevin to the light and throw the Horde into chaos, he crushes her to death against him. He then proceeds to rip apart and eat both Marcia and Claire. Meanwhile, Casey stumbles upon Dr. Fletcher's corpse, as well as an instructional note the doctor the left behind on how to defeat the Beast. When the Beast closes in on her, demonstrating the ability to climb up walls, Casey uses Kevin's real name to call him to the light. The Beast is briefly tamed, and the actual Kevin (who has been restrained for years by the other personalities, and has no memory of what he has done between then and now) emerges. Horrified by the death of Dr. Fletcher, he orders Casey to kill him with a shotgun he bought before the Beast can return—however, the Beast returns when Kevin's other personalities desperately try to defend themselves. As the Beast reawakens, it pursues Casey into the basement of its lair, eventually cornering her in a cell. Casey shoots him twice at point blank range with Kevin's shotgun, but it proves to be ineffective, as the shells fail to do more than pierce his skin.
Reaching the cell, the Beast voices his maniacal plans to rid the world of the 'untouched' (As in, the people who he feels have never suffered pain or discomfort, unlike him). The Beast bends the bars with its bare hands to gain entry, but before finishing, it notices the scars on her body from her previous abuse at the hands of her uncle. Realizing that he has met a kindred spirit, one who has also suffered exceptional abuse at the hands of a trusted relative, he declares her to be "pure," and leaves a bewildered Casey in peace. Casey is eventually discovered and rescued by Kevin's co-workers at the Philadelphia Zoo. Recovering from the battle, Patricia, Dennis and Hedwig speculate in awe at the power of the Beast, marvelling at what it was shown to be capable of doing. Kevin voices his plans to change the world, and awaits the next feeding of 'the Beast' now under the man's newest and diabolical collective nom de guerre "The Horde".